In a medium saucepan, melt butter over medium heat. Stir in marshmallows and cook, stirring, until smooth, 2 minutes; remove from heat. In a bowl, combine marshmallows with cereal; mix well. In the bottom of a 9" x 13" pan, spread half of cereal mixture into a thin, even layer. Top with warmed chocolate. Place graham crackers in a single layer over chocolate, breaking crackers as needed to fit; top with crème. Spread remaining half of cereal mixture over crème; press lightly to cement layers together. Let s'mores sit at room temperature for at least 15 minutes before cutting and serving.